The present invention relates to an image sensor, and more particularly, to a backside illuminated image sensor, which can be applied to an active pixel sensor (APS) such as a complementary metal oxide semiconductor (CMOS) image sensor or a charge coupled device (CCD) image sensor.
An image sensor is a semiconductor device that converts an optical image into an electrical signal. The image sensor includes a light receiving part (generally referred to as a photodiode) for sensing light, and a logic circuit part for processing the sensed light into electrical signals.
A pixel of an image sensor includes a photodiode for receiving light to generate photoelectric charges, and a charge transfer gate for transferring the photoelectric charges to a sensing node of the pixel.
A conventional image sensor has a front illumination configuration, in which a photodiode is formed below a substrate surface and logic circuits are formed on the substrate, so that light is illuminated on the top surface of the substrate. However, the photo response characteristic of the photodiode is poor because multiple upper layers formed on the photodiode cause light loss. Also, it is difficult to covert incident light flux into photoelectric charges because the penetration depth of photons is large.
To overcome these limitations, a backside illuminated image sensor has been proposed which illuminates a substrate from the rear surface thereof.
FIG. 1 is a sectional view of a conventional backside illuminated image sensor disclosed in US Patent Publication No. 2006-0068586A1.
Referring to FIG. 1, an n-well 120 serving as a junction cathode is formed on a p-type silicon 130 by performing a predetermined process on a silicon-on-insulator (SOI) wafer having a silicon/buried oxide/silicon structure, and a logic circuit (not shown) and metal lines 150 are formed thereon. A supporting substrate 140 is attached, and the silicon on the rear surface of the SOI wafer is polished until the buried oxide layer. An anti-reflecting layer 220 and a microlens 230 are formed on the resulting structure. Therefore, photons are incident from the rear surface of the substrate. The conventional backside illuminated image sensor further includes a p-type ion injection region 125 for preventing cross-talk, and first and second insulation layers 160A and 160B.
However, a metal reflector 240 must be separately provided at a position corresponding to the photodiode, in order to reduce signal loss caused when long-wavelength light passes through the silicon (substrate) whose thickness is reduced. Therefore, a metal process must be added or limitations must be imposed on a metal layout.
Additionally, in the convention backside illuminated image sensor, the internal potential of the photodiode is determined according to processing conditions (doping concentrations, depths, etc.) and the width of the depletion region is determined. Accordingly, when a depletion region is not formed near the rear surface of the substrate, the cross-talk is generated. That is, photoelectric charges generated around the rear surface of the substrate do not reach the depletion region of the photodiode and move to a neighboring pixel. Also, sensitivity to short wavelength is poor.